This invention relates to boron-containing red light-emitting phosphors and to light sources incorporating such phosphors.
A phosphor is a luminescent material that absorbs radiation energy in a portion of the electromagnetic spectrum and emits energy in another portion of the electromagnetic spectrum. Phosphors of one important class are crystalline inorganic compounds of high chemical purity and of controlled composition to which small quantities of other elements (called “activators”) have been added to convert them into efficient fluorescent materials. With the right combination of activators and inorganic compounds, the color of the emission can be controlled. Most useful and well-known phosphors emit radiation in the visible portion of the electromagnetic spectrum in response to excitation by electromagnetic radiation outside the visible range. Well-known phosphors have been used in mercury vapor discharge lamps to convert ultraviolet (“UV”) radiation emitted by the excited mercury vapor to visible light. Other phosphors are capable of emitting visible light upon being excited by electrons (used in cathode ray tubes) or X rays (for example, scintillators in X-ray detection systems).
Fluorescent lamps having high luminous output and color rendering indices (“CRI”), which are based on mercury discharge and used for illumination, typically include three phosphors that convert UV radiation of the mercury discharge into relatively narrow bands of blue, green, and red visible light, concentrated in the spectral regions where the human eye has the highest sensitivity (450, 540, and 610 nm). Europium-activated yttrium oxide (Y2O3:Eu3+) has been a favorite red light-emitting phosphor, having a peak emission at about 613 nm. However, the manufacture of this phosphor requires a high-purity Y2O3 because impurities, such as iron, in Y2O3 tend to act as competing absorbing centers for the 254 nm radiation. Other red light-emitting phosphors have also been used, but each has a certain drawback. 6MgO.As2O5:Mn2+ and 3.5MgO0.5MgF2.GeO2:Mn4+ have peak emission in the deep red region at about 655 nm, reducing the luminous output of light sources using these phosphors. In addition to having an emission peak at about 630 nm, GdMgB5O10:Ce3+,Mn2+ emits broadly in the wavelength range of 580–700 nm. Thus, this phosphor does not easily provide a high CRI. Similarly, (Sr,Mg)3(PO4)2:Sn2+ has an emission peak at 630 nm, and emits even more broadly than GdMgB5O10:Ce3+,Mn2+ , from about 540 nm to about 720 nm. A more recent addition to the red light-emitting phosphors is YVO4:Eu3+ , which has two peaks at about 607 nm and 619 nm. However, this phosphor also has a substantial emission at about 592 nm. In addition, residual, unreacted V2O5 in this phosphor lowers its light output. Other red light-emitting phosphors, such as GdAlO3:Eu3+ and Y2O2S:Eu3+ have been proposed for light sources. However, the long-term stability of these phosphors in a mercury discharge needs to be improved.
Therefore, there is a continued need for new red light-emitting phosphors that emits in a narrow band near 610 nm, at which the human eye is more sensitive, and that does not impose special requirements in their manufacture. It is also very desirable to use such red light-emitting phosphors to produce light sources having high CRIs.